It's rather simple if you follow the steps laid out on the web page.
Start with the "Tonearm/Turntable Database" (http://www.cartridgedb.com/default_arms.asp) which you'll find on the main page of the link. Put in the information you have for your tonearm; manufacturer and model will be sufficient if your arm is in the database. Keep this information handy as you open another window to reach the "Cartridge Database" (http://www.cartridgedb.com/) which is the main link in this post.
On the Caratridge page find your information for the specific cartridge you are interested in. Do this by inserting your cartridge manufacturer and the model, once again this amount of information should be sufficient if your cartridge is in the database. Keep this page open while you move to one of two places, either "Cartridge Resonance Evaluator" by clicking on the tab or "Cartridge Database Tools" which you can find at the top of the "Cartridge Database" page.
Follow directions after that. On the "Cartridge Database Tools" page you'll find the simplest way to get the results you want with decimal point accuracy. Just insert your arm's effective mass in grams in the "Tonearm Effective Mass" slot and your cartridge's mass in the slot for "Cartridge Mass" and its compliance in the slot provided for "Compliance". This provides a simple read out that is to a decimal point accurate.
To check this and get a better idea of the range your tonearm will accept in cartridge compliance use the "Cartridge Resonance Evaluator". This will provide a graph with horizontal and vertical axis that might be more useful for oddball arms or cartridges.
For example, my RB300 arm has an effective mass of 11.5 grams while the Grado Sonata has a mass of 6.5 grams with a compliance of 20 X 10-6 cm/Dyne. Together they yeild a resonance of 8.38Hz. Pretty close to peachy keen!